This invention relates to soldering and, more particularly, to an improved method and apparatus for soldering or tinning printed circuit boards by passing the same through a standing wave of molten solder.
A known technique for processing printed circuit boards involves the tinning or coating of the printed circuits with solder, by passing the boards with their printed circuit surfaces in contact with a standing wave of molten solder. The boards may also be fluxed, before passing through the standing wave of molten solder, by passing the boards through a standing wave of liquid flux. The standing wave of molten solder is produced by causing an upward flow of molten solder through a nozzle having a substantially rectangular discharge opening extending laterally of the wave, so that the molten solder overflows the edges of the nozzle and returns to a solder tank. The wave may be either one-sided, in which case it overflows only one longer edge of the nozzle, or it may be double-sided, in which case it overflows both longer edges of the nozzle discharge opening.
A molten solder wave, in the same manner as any free flowing liquid material, develops a pattern similar to that of a water jet discharged under pressure from a nozzle. Depending on the initial angle of the nozzle, the water jet will either initially travel horizontally or initially travel upwardly but, under the influence of gravitational forces, the stream of water tends to flatten and then to drop with ever increasing speed. That is, it is accelerated downwardly by gravity so that the shape of the water jet is the same as a ballistic line having a similar initial velocity and angle. As the flow quantity is equal at any point along the line, with increasing speed, the cross-sectional area of the flowing stream decreases.
Soon after the introduction of wave soldering for processing printed circuit boards, various types of so-called wave formers were used to extend the standing wave. Soldering was effected further on, on the top of the wave, but that part of the wave which was not in contact with the printed circuit board imparted some preheating and/or after heating, by radiation, to the board, which facilitated icicle-free soldering.
However, the application of these wave formers did not change the shape of the upper or active surface of the wave actually engaged in the soldering operation, which still maintained a ballistic curvature under the influence of gravitational forces. With the exponential increase in the number of printed circuit boards to be soldered, the soldering process, over the years, has had to be accelerated or speeded up.
Despite the fact that the best soldering is produced when the solder wave just touches the printed wiring, it was quite common to attempt to elongate the contact length, between the solder and the printed circuit board, by pressing the board into the wave to flatten the wave top or upper surface. In this procedure, longer contact time and higher speeds were achieved, but accompanied with the formation of icicles. This compromises the results, but the results were accepted because no better way was available.
When the weight of electronic assemblies turned to be important, especially in the aerospace industry, inclined transfer application came into use because, at the exit point from the wave, the solder areas were drained better with respect to adhering solder, thus leaving less excess solder on the board.
With all known arrangements, it has been difficult to obtain more than a point contact between the peak of the wave and the surface of a printed circuit board and, as mentioned, the width of the contact band, actually the length of contact between the solder wave and the board in the direction of travel, generally has been increased only by pressing the board into a wider surface contact with the solder wave. This results, among other things, in an unnecessarily heavy coating of the solder on the printed circuit board connections. Furthermore, pressing a board into the wave to some extent in order to flatten the wave for a distance of the order of maybe 1 or 2 inches, results in the normally parabolic shape of the wave assuming a mushroom curvature, which interferes with draining of the solder from the trailing edge of the board passing through the wave and results in solder icicles depending from the board.
In this connection, it should be emphasized that one of the important factors in connection with wave soldering is to maintain the exit angle, that is the angle between the under-surface of the board and the flowing solder, as small as possible in order to maintain heat on the "draining" end of the board to prevent formation of icicles of solder. None of the prior art arrangements providing a more extended contact of the board with the molten solder wave by depressing the board into the wave are effective to prevent formation of such icicles.